This invention relates to a method for making carbonaceous materials that contain one or more ferrous metal components. More particularly, the invention relates to a fluidized bed method for disproportionating carbon monoxide using a particulate ferrous group metal component catalyst in a fluidized bed that contains a sufficient amount of inert abradant to remove a substantial amount of the carbon formed on the surface of the particulate catalyst.
U.S. patent application Ser. No. 99,789 filed Dec. 3, 1979, discloses a new family of carbon/metal materials and methods for making them. The entire disclosure of that application is hereby incorporated in this application by reference. Briefly, that application discloses making carbon/metal materials by disproportionating carbon monoxide in the presence of a ferrous group metal component catalyst which may be a metal, an alloy, a carbide or other metallic substance. As explained there, disproportionation means any of the reactions which occur in the presence of a ferrous group metal to produce carbon from carbon monoxide, which can be part of a mixture containing hydrogen or other substances. The following are typical reactions: ##STR1##
In these processes, carbonaceous material forms on and grows from the catalyst surface primarily in the form of fibers. Some non-fibrous carbon can also be present. As disproportionation continues, these fibers become tangled masses that occupy increasingly larger volumes in the fluidized bed reactor. Simultaneously, the effective density of the carbonaceous material produced falls toward the range of about 0.05 to about 0.7 grams per cubic centimeter.
It is difficult to produce fibrous carbon/metal materials of preselected properties in a continuous process. For example, as substantial amounts of carbon deposit on catalyst particles larger than 120 microns, the bed tends to form two substantially distinct parts. The lower part of the bed includes relatively large, partially carburized ferrous group metal component particles. The upper part of the bed includes smaller carbon and ferrous metal component particles that have broken away from the larger particles in the lower part of the bed. If the bed contains only carbon and ferrous metal component catalyst, the bed mass generally becomes larger than desired, and the ratio of ferrous metal to carbon in the elutriated product is not easily controlled. Further, if the superficial gas velocity is too low, the bed mass can increase until the entire reactor is filled with carbon/metal material, which is undesirable, and can also result in too broad a range of carbon to metal ratio in the product, too high a carbon to metal ratio in the product, and too high bulk density of the product. If the superficial gas velocity is too high, then material can be entrained from the reactor with a lower than desired carbon to metal ratio.
Other problems can arise as a result of overall or local temperature fluctuations in the bed. As the bed temperature is increased, the ratio of nonfibrous to fibrous carbon in the carbon/metal product can become larger than desired. Control of temperature is difficult because the carbon deposition process is exothermic, so that in a practical commercial system it is necessary to have heat exchange tubes immersed in the fluid bed to provide heat removal to maintain a desired bed temperature. Low density carbon/metal material can cling to heat transfer surfaces, resulting in an insulating layer being formed on the heat transfer surfaces which prevents adequate heat removal and results in an undesirable rise in the bed temperature.
Thus, there is a need for a continuous process for efficiently preparing fibrous carbon/metal materials of preselected properties.